1. Field of the Invention
Embodiments of the present invention relate, in general, to distance measuring equipment and particularly to reducing the effects of systematic error in the analog to digital conversion of detected laser pulses.
2. Relevant Background
Laser range finding apparatuses determine distance measurements based on a round trip calculation of the time of flight of laser pulses. The detection of reflected laser pulses depends on photodiodes, and the two major categories of receiver photodiodes used in laser detection are P-type Intrinsic N-type (“PIN”) diodes and avalanche photodiodes (“APD”).
When using time of flight to measure distances, the time of flight is the time needed for a laser pulse to travel from the laser range finder to the target and back. Here, the term “reflected pulse” means a laser pulse that has been reflected from a target, and the term “detected pulse” means a reflected pulse that has been detected at a detection unit, i.e. a receiver photodiode. For a pulse to be detected it must be of sufficient intensity to distinguish itself from noise.
While this concept is theoretically simple, in practice it is more difficult to obtain readings due to the variability of a number of factors. For example, different targets can have different surface characteristics such as color and can be positioned in different environments having different backgrounds. Different colors and backgrounds can affect the intensities of the reflected laser pulses. Therefore, even when the distances from the laser range finder to a first target and to a second target are identical, the intensity of the detected pulses from the first and second targets may be substantially different.
Most long distance laser range finders utilize APDs. APDs possess an electro-optic gain which eliminates or significantly reduces the effect of noise produced by the receiver. A PIN diode does not provide such an electro-optic gain thus requiring the received signal to distinguish itself over an often substantial amount of noise. Thus, while the PIN diode functions adequately for short range returns wherein the signal to noise ratio is relatively high, long range detection has long relied on APD based detectors. Unfortunately, APDs are substantially more expensive than PIN diodes. Thus, a design tradeoff is often made between the competing factors of cost and performance.
Theoretically, the noise associated with a PIN diode based detection circuit can be accommodated for by firing more pulses and summing the results. Random noise is proportional to the summation of the square root of the number of pulses while the actual laser return signal varies linearly with the number of pulses. Accordingly, the more pulses you fire the better the signal to noise ratio. However, in practice, a limit is placed on this approach by resolution of the Analog to Digital Converter (“ADC”) and systematic noise induced into the system that is linked to the clock running the ADC. Thus, the limit of using PIN diodes has been associated with a combination of the ADC resolution and systematic noise correlated with the clocking frequency that is running the ADC.
Systematic noise in ADCs is synchronized with the edge of the clock pulse driving the ADC. This type of noise is well known to one skilled in the relevant art and is often regarded as insignificant as compared to other sources of error such as random noise. However, as other sources of noise are addressed, this systematic noise source becomes problematic. And, while error linked with the running of a clock found in a processor or an Application Specific Integrated Circuit (“ASIC”), and the like can be shielded from the detection circuit, systematic error associated with an ADC remains a challenge. ADC systematic error is induced by the actual working components of the ADC. ADC designs include switches that alternate between multiple parallel pathways to convert analog signals to digital signals. This switching is linked to the clock frequency and other sub-harmonic clock frequencies resulting in systematic noise.
These errors act to mask the detection of incoming signals in PIN diodes. While the use of certain ADCs can overcome many of these problems, the cost associated with such a solution is prohibitive. Reducing the effects of correlated noise, spurious signals, and systematic clock errors associated with an ADC is desirable.